Insulated-gate bipolar transistors (IGBTs) are power semiconductor devices primarily used as electronic switches, and in more recent devices are noted for combining high efficiency and fast switching. An IGBT combines the simple gate-drive characteristics of a metal oxide semiconductor field effect transistor (MOSFET) with the high-current and low-saturation-voltage capability of a bipolar transistor. The IGBT comprises an isolated gate field effect transistor (FED for the control input and a bipolar power transistor for a switch in a single device. IGBTs are often used in medium- to high-power applications such as switched-mode power supplies, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities and high blocking voltages.
When an IGBT is used in a half bridge configuration, a negative voltage is required on its gate when a low side driver device is switched on in order to avoid cross conduction between high and low side driver devices. It is known to use a series capacitor topology to enable such a negative IGBT gate voltage to be generated using only a single positive supply.
FIG. 1 illustrates a simplified circuit diagram of an IGBT driver module 105 comprising a known series capacitor topology for driving an IGBT device 100. The IGBT device 100 is driven through a series capacitance Cs 110 by a high side driver device 130 and a low side driver device 140. The series capacitance Cs 110 comprises a value of, say, 10 uF. By contrast, a gate capacitance Cg (not shown) of the IGBT device 100 comprises, say, approximately 100 nF. As such, the series capacitance Cs 110 is approximately 100 times larger than the gate capacitance Cg of the IGBT device 100. As the series capacitance Cs 110 is in series with the gate 102 of the IGBT device 100, the series capacitance Cs 110 is charged and discharged along with the gate capacitance Cg of the IGBT device 100. So for a 25V voltage variation on the gate capacitance Cg of the IGBT device 100, a 0.25V voltage variation is required across the series capacitance Cs 110 (a ratio of 100:1).
In order to achieve the required negative voltage on the gate 102 of the IGBT device 100 when the low side driver device 140 is switched on (required in order to avoid cross conduction between the high and low side driver devices 130, 140), the series capacitance 110 is pre-charged to achieve a voltage there across of approximately 10V. As such, the (pre-charged) series capacitance 110 may be considered as a 10V source voltage that shifts the gate voltage of the IGBT device 100 by approximately −10V.
In FIG. 1, the IGBT driver module 105 comprises a pre-charge component arranged to pre-charge the series capacitance Cs 110. The pre-charge component illustrated in FIG. 1 comprises a Zener diode 150 controllable (via switch 152) to force a first node 112 of the series capacitance Cs distal to the gate 102 of the IGBT device 100 to its breakdown voltage (Vz) of, say, a voltage of 10.6V. The pre-charge component illustrated in FIG. 1 further comprises a closed loop amplifier 154 controllable (via switch 156) to force a second node 114 of the series capacitance operably coupled to the gate 102 of the IGBT device 100 to a voltage ΔV, where ΔV is equal to the required voltage variance across the series capacitance Cs to achieve the desired voltage variance across the gate capacitance Cg. Thus, for the case described above where, for a 25V voltage variation on the gate capacitance Cg of the IGBT device 100, a 0.25V voltage variation is required across the series capacitance Cs 110 (a ratio of 100:1), ΔV=0.25V. In this manner, the pre-charge component is arranged to pre-charge the series capacitance Cs 110 such that it comprises a voltage there across of Vz−ΔV=10.6V−0.25V=10.35V.
The IGBT driver module 105 further comprises a high side drive cut-off component arranged to switch off a high side driver device 130 when a required voltage increase has been achieved at the gate 102 of the IGBT device 100, i.e. in the above case a 25V voltage increase. To detect such a voltage increase, the high side drive cut-off component comprises a comparator 160 arranged to compare the voltage across the series capacitance Cs 110 to a reference voltage, which in the illustrated example is provided by the Zener diode 150, and cause the high side driver device 130 to be ‘switched off’ (i.e. to stop forcefully driving the gate of the IGBT device 100) when the voltage across the series capacitance Cs 110 reaches the reference voltage.
As such, the comparator 160 compares the voltage across the series capacitance Cs 110 to the 10.6V breakdown voltage of the Zener diode 160 and switches off the high side driver device 130 when the voltage across the series capacitance Cs 110 reaches 10.6V. As previously mentioned, the series capacitance Cs 110 is pre-charged to 10.35V. As a result, the comparator 160 is arranged to switch off the high side driver component 130 once a required 0.25V voltage increase has been achieved across the series capacitance Cs 110. In this manner, the high side driver component 130 may be arranged to forcefully drive the gate 102 of the IGBT device 100 to achieve the 25V voltage increase thereof in a relatively short time, with the comparator 160 switching off the high side driver component 130 once the required voltage increase has been achieved.
A problem with such a technique for driving the gate 102 of an IGBT device 100 is that it requires accurate matching between the Cs:Cg ratio (i.e. the ratio between the series capacitance size and the gate capacitance size) and the variance in voltage across the series capacitance permitted before the high side driver device 130 is switched off. However, the capacitances of the series capacitance Cs 110 and the gate capacitance for the IGBT device 100 can each vary by +/−15%, which can have a significant effect on the Cs:Cg ratio, and thus the relationship between the voltage variance across the series capacitance Cs 110 and the corresponding voltage variance across the gate capacitance of the IGBT device 100. Specifically, if the ratio is too low, for example 85:1 instead of 100:1, a voltage variance of 0.25V across the series capacitance Cs 110 will only achieve a voltage variance across the gate of the IGBT device 100 of 21.25V, and not the required 25V.